Information recording and reproduction apparatus carrying out recording and reproduction of information using laser beam

ABSTRACT

An information recording and reproduction apparatus for a magneto-optical medium includes a synchronizing signal generation circuit for generating a synchronizing signal to provide a pulsed laser beam at the time of reproduction according to an internal clock obtained from a reproduction signal. The laser beam is driven on/off according to the synchronizing signal at the time of reproduction to have the laser spot on the signal plane prevented from being enlarged, so that the reproducible domain is smaller than the domain of when a laser beam is continuously emitted. In reproduction, an optical superresolution method is applied simultaneous to provision of a pulsed laser beam to allow reproduction of higher density. An external synchronizing signal generation circuit for generating an external synchronizing signal according to a wobble on a surface of a recording medium can be used instead of a synchronizing signal generation circuit according to an internal clock.

This application is a division of prior application Ser. No. 08/861,791,filed May 22, 1997, now U.S. Pat. No. 6,240,056.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to information recording and reproductionapparatuses for recording and reproducing information using a laserbeam. More particularly, the present invention relates to an informationrecording and reproduction apparatus that can reproduce at high densityinformation from an information recording medium such as amagneto-optical recording medium and a phase transition recording mediumby emitting a pulsed laser beam in reproduction.

2. Description of the Background Art

A magneto-optical recording medium is noteworthy of its rewritableability, large storage capacity, and high reliability. It is alreadyavailable for practical usage as the memory and the like for computers.However, the recording and reproducing technique of information athigher density is required in accordance with increase in the amount ofinformation to be recorded and reduction in the size of the recordingand reproduction apparatus.

The technique for recording and reproducing information at higherdensity is divided into the technique at the recording and reproductionapparatus end and the technique of the recording medium end.

The former includes, in addition to the method of rendering thewavelength of a laser beam shorter, the so-called opticalsuperresolution method. This method achieves a focused spot that exceedsthe diffraction limit of a laser beam by inserting a light blockingobject in the light path of the laser beam. This optical superresolutionmethod is disclosed in “High Density Optical Recording bySuperresolution”, Japanese Journal of Applied Physics, Vol. 28,Supplement 28-3, pp. 197-200, 1989 by Y. Yamanaka et al., for example.

The latter technique includes, in addition to the method of narrowingthe pitch of the recording track of the medium, the method of improvingthe reproducing resolution using a magnetic multilayer film. This art ofimproving reproduction resolution using a magnetic multilayer filmincludes the step of providing a magneto-optical recording medium with amagnetic multilayer film having a recording layer and a reproductionlayer. By taking advantage that the temperature distribution within thelaser spot exhibits a Gaussian distribution that takes the maximum valuearound the center thereof, the magnetized state of the recording layeris selectively transferred to the reproduction layer by virtue of theexchange-couple force when being irradiated with a laser beam forreproduction. The magnetized state of that reproduction layer is readout at high density with a laser beam of a light power that is lowerthan that of recording. This technique is disclosed in, for example,“Recent Progress in Magnetically Induced Superresolution”, Proceedingsof Magneto-Optical Recording International Symposium '96, Journal ofMagnetics Society of Japan, Vol. 20, Supplement No. S1, pp. 7-12, 1996by M. Kaneko et al.

Conventional reproduction of information from such a magneto-opticalrecording medium is effected by continuously emitting a laser beam onthe recording surface of a medium. However, there was a problem thatreproduction at high density cannot be implemented when the laser beamfor reproduction is emitted continuously.

This problem will be specifically described hereinafter. By using theoptical superresolution method, the diameter of the laser beam forreproduction can be reduced up to approximately 0.78 μm at the presentstage. The shortest domain length that can be reproduced with this beamdiameter is 0.4 μm beam which corresponds to approximately half thediameter. At the current stage, the shortest domain length that can berecorded has become as small as approximately 0.15 μm. There was aproblem that, even if the optical superresolution method is used,information recorded at a domain smaller than 0.40 μm cannot bereproduced. The reproduction technique cannot follow the currentlyavailable high density recording.

This problem is due to the fact that the laser beam is continuouslyemitted at the time of reproduction. This continuous emission of a laserbeam will result in a greater laser spot for reproduction on the signalsurface due to the fact that heat diffusion is generated on the signalsurface of the medium and that the laser spot on the signal surface isextended by the relative movement between the medium and the laser beam.As a result, information recorded in a small domain cannot bereproduced.

This continuous emission of a laser beam also induces the problem thatthe laser output is gradually reduced since the semiconductor laserwhich is the light source is constantly turned on during reproduction.The lifetime of the semiconductor laser is also shortened and powerconsumption is increased by this continuous usage.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide an information recording and reproduction apparatus that canreproduce at high density information using a laser beam from aninformation recording medium on which information is recorded at highdensity.

Another object of the present invention is to provide an informationrecording and reproduction apparatus that does not have the output of asemiconductor laser which is a light source reduced.

A further object of the present invention is to provide an informationrecording and reproduction apparatus that allows a longer lifetime of asemiconductor laser which is a light source and that allows powerconsumption.

According to an aspect of the present invention, an informationrecording and reproduction apparatus for an information recording mediumincludes an optical head, an information reproduction circuit, a drivecircuit, and a first synchronizing signal generation circuit. Theoptical head emits a laser beam on a plane of an information recordingmedium on which a signal is recorded and detects the laser beamreflected from the signal recording plane. The information reproductioncircuit reproduces information from the laser beam detected by theoptical head. The drive circuit drives the optical head so as to renderthe emitted laser beam into a pulsed laser beam. The first synchronizingsignal generation circuit generates and provides to the drive circuit afirst synchronizing signal for emitting the pulsed laser beam on thesignal recording plane in reproduction. The first synchronizing signalis generated in synchronization with a signal reproduced from the signalrecording plane by the pulsed laser beam.

According to another aspect of the present invention, an informationrecording and reproduction apparatus for an information recording mediumincludes an optical head, an information reproduction circuit, a drivecircuit, and a first synchronizing signal generation circuit. Theoptical head emits a laser beam on a plane of an information recordingmedium on which signal is recorded and detects the laser beam reflectedat the signal recording plane. The information reproduction circuitreproduces information from the laser beam detected by the optical head.The drive circuit drives the optical head so as to render the emittedlaser beam into a pulsed laser beam. The first synchronizing signalgeneration circuit generates and provides to the drive circuit a firstsynchronizing signal for emitting the pulsed laser beam on a signalrecording plane in reproduction. The first synchronizing signal isgenerated in synchronization with a signal reproduced from the signalrecording plane by the pulsed laser beam. The optical head is controlledso that a laser beam formed of a main lobe and side lobes is emitted ona signal recording plane by having the inner portion blocked of thelaser beam emitted from the optical head only in reproduction.

According to a further aspect of the present invention, an informationrecording and reproduction apparatus for an information recording mediumhaving a reference information signal of a predetermined cycle recordedin advance on the surface includes an optical head, an informationreproduction circuit, a drive circuit, and an external synchronizingsignal generation circuit. The optical head emits a laser beam on aplane of the information recording medium where a signal is recorded anddetects the laser beam reflected from the signal recording plane. Theinformation reproduction circuit reproduces information from the laserbeam detected by the optical head. The drive circuit drives the opticalhead so as to render the emitted laser beam into a pulsed laser beam.The external synchronizing signal generation circuit generates andprovides to the drive circuit an external synchronizing signal foremitting the pulsed laser beam on a signal recording plane inreproduction. The external synchronizing signal is generated insynchronization with a reference information signal reproduced from theinformation recording medium by a pulsed laser beam.

According to a further aspect of the present invention, the informationrecording and reproduction apparatus further includes a duty correctioncircuit for correcting the duty of a synchronizing signal.

According to still another aspect of the present invention, aninformation recording and reproduction apparatus further includes asecond synchronizing signal generation circuit to delay a synchronizingsignal for a predetermined time period to generate and provide to theinformation reproduction circuit a second synchronizing signal. Theinformation reproduction circuit reproduces information insynchronization with the second synchronizing signal.

A main advantage of the present invention is to suppress increase of alaser spot on a signal plane of a medium by emitting a pulsed laser beamin reproduction to allow reproduction of information from a smallerdomain.

Another advantage of the present invention is to suppress reduction inlaser output, allow increase in the lifetime of the semiconductor laser,and allow reduction in power consumption by a pulse drive of thesemiconductor laser at the time of reproduction.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an example of a stacked layer structure ofa magnetically induced superresolution magneto-optical recording mediumused in embodiments of the present invention.

FIG. 2 is a sectional view of an example of a stacked layer structure ofa general magneto-optical recording medium used in embodiments of thepresent invention.

FIG. 3 is a sectional view of another example of a stacked layerstructure of a magneto-optical recording medium used in embodiments ofthe present invention.

FIG. 4 is a sectional view of an example of a stacked layer structure ofa phase transition recording medium used in embodiments of the presentinvention.

FIG. 5 is a block diagram showing an entire structure of an informationrecording and reproduction apparatus according to a first embodiment ofthe present invention.

FIGS. 6A-6C are diagrams for describing the relationship of a magneticfield, a pulse laser beam, and a record domain.

FIGS. 7A-7C are diagrams showing the relationship of a record domain, areproduction laser beam, and a reproduction signal waveform when a laserbeam is continuously emitted in reproduction.

FIGS. 8A-8D are diagrams showing the relationship of a record domain, apulse laser beam, a pulse reproduction waveform, and a reproductionsignal waveform when a pulsed laser beam is emitted in reproduction.

FIG. 9 is a block diagram showing a modification of the informationrecording and reproduction apparatus of the first embodiment shown inFIG. 5.

FIGS. 10A-10C are diagrams showing the relationship of a record domain,a pulse reproduction waveform, and a sample waveform when a pulsed laserbeam is emitted in reproduction.

FIGS. 11A-11D are diagrams showing reproduction signal waveforms ofa-general magneto-optical disk and a magnetically inducedsuperresolution magneto-optical disk when a pulsed laser beam is emittedin reproduction.

FIGS. 12A-12C are diagrams showing reproduction signal waveforms from alarge domain and a small domain of a magnetically inducedsuperresolution magneto-optical disk when a pulsed laser beam is emittedin reproduction.

FIG. 13 is a diagram showing the relationship between the CN ratio anddomain length when a pulsed laser beam is emitted in reproduction.

FIG. 14 is a block diagram showing a structure of an optical system usedin an information recording and reproduction apparatus according to asecond embodiment of the present invention.

FIGS. 15A and 15B are diagrams showing the structure and driving statesof the polarizing filter and polarization plane rotary unit of theoptical system shown in FIG. 14.

FIG. 16 is a schematic diagram showing the characteristics of apolarizing filter shown in FIGS. 15A and 15B.

FIGS. 17A and 17B show another driving states of the polarization planerotary unit of FIGS. 15A and 15B.

FIGS. 18A and 18B show another structure and driving states of thepolarization plane rotary unit of the optical system of FIG. 14.

FIGS. 19A and 19B show a structure and driving states of a liquidcrystal shutter as an alternative to the polarizing filter of theoptical system shown in FIG. 14.

FIGS. 20A and 20B are front views of a glass polarizer as an alternativeto the polarizing filter of the optical system shown in FIG. 14.

FIG. 21 is a diagram for describing the operation principle of a Pockelscell as an alternative to the TN type liquid crystal of the opticalsystem shown in FIG. 14.

FIG. 22 is a diagram for describing the operation principle of a Faradaycell as an alternative to the TN type liquid crystal of the opticalsystem shown in FIG. 14.

FIG. 23 is a block diagram showing a structure of an optical system usedin an information recording and reproduction apparatus according to athird embodiment of the present invention.

FIGS. 24A and 24B show a structure and driving states of the polarizingfilter and the polarization plane rotary unit of the optical systemshown in FIG. 22.

FIG. 25 is a diagram showing the relationship between a laser beam forreproduction and jitter at the time of reproduction according to thesecond and third embodiments of the present invention.

FIG. 26 is a schematic diagram showing an example of a planeconfiguration of a wobble formed on a surface of an informationrecording medium.

FIG. 27 is a block diagram showing an entire structure of an informationrecording and reproduction apparatus according to a fourth embodiment ofthe present invention.

FIG. 28 is a schematic diagram showing a signal detection principle inthe optical head shown in FIG. 27.

FIGS. 29A-29D are timing charts for describing an operation of theexternal synchronizing signal generation circuit of FIG. 27.

FIG. 30 is a block diagram specifically showing the externalsynchronizing signal generation circuit of FIG. 27.

FIGS. 31A-31F are timing charts for describing an operation of theexternal synchronizing signal generation circuit of FIG. 30.

FIGS. 32A-32C are timing charts for describing an operation of thesynchronizing signal input circuit of FIG. 27.

FIGS. 33A-33C are timing charts for describing the necessity of anexternal synchronizing signal.

FIGS. 34, 35 and 36 are block diagrams showing a first modification, asecond modification, and a third modification, respectively, of thefourth embodiment shown in FIG. 27.

FIGS. 37A-37B are timing charts for describing an operation of thesecond synchronizing signal generation circuit of FIG. 36.

FIGS. 38, 39, and 40 are block diagrams showing a fourth modification, afifth modification, and a sixth modification, respectively, of thefourth embodiment of FIG. 27.

FIGS. 41A-41C are schematic diagrams showing examples of a planeconfiguration of a fine clock mark formed on a surface of an informationrecording medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to an information recording andreproduction information that reproduces information at high densityfrom a recording medium such as a magneto-optical recording medium or aphase transition recording medium on which information is recorded athigh density. First, specific examples of a magneto-optical recordingmedium and a phase transition recording medium used in the informationrecording and reproduction apparatus of the embodiments of the presentinvention will be described.

An example of a stacked layer structure of a magnetically inducedsuperresolution magneto-optical recording medium used in the embodimentsof the present invention will be described with reference to FIG. 1. Themagnetically induced superresolution magneto-optical medium disk used inthe embodiment is a magneto-optical recording medium that allows highdensity recording, including a magnetic multilayer film formed of arecording layer and a reproduction layer.

Referring to FIG. 1, the magnetically induced superresolutionmagneto-optical recording medium includes a substrate 1 formed of atransmissive polycarbonate, glass, and the like, a first dielectriclayer 2 formed of SiN on substrate 1, a reproduction layer 3 formed ofGdFeCo on first dielectric layer 2, a recording layer 4 formed of TbFeCoon reproduction layer 3, a second dielectric layer 5 formed of SiN onrecording layer 4, a heat radiating layer 6 formed of a metal having ahigh thermal conductivity such as Al on second dielectric layer 5, and aprotection layer 7 formed of ultraviolet curing resin on heat radiatinglayer 6.

First and second dielectric layers 2 and 5 each have a film thickness of800 Å(tolerable error ±10 Å). Reproduction layer 3 has a film thicknessof 10000 Å(tolerable error ±10 Å). Recording layer 4 has a filmthickness of 500 Å(tolerable error ±10 Å). Heat radiating layer 6 has afilm thickness of 200 Å(tolerable error ±10 Å). Protection layer 7 has afilm thickness of 10 μm (tolerable error ±1 μm).

An example of a stacked layer structure of a general magneto-opticalrecording medium which is not a magnetically induced superresolutionmagneto-optical recording medium, used in the embodiment of the presentinvention will be described with reference to FIG. 2.

Referring to FIG. 2, the general magneto-optical recording mediumincludes a substrate 1, a first dielectric layer 2 formed of SiN onsubstrate 1, a recording-reproduction layer 8 formed of TbFeCo on firstdielectric layer 2, a second dielectric layer 5 formed of SiN onrecording-reproduction layer 8, a heat radiating layer 6 formed of ametal of high thermal conductivity such as Al on second dielectric layer5, and a protection layer 7 formed of ultraviolet curing resin on heatradiating layer 6.

According to the structure shown in FIG. 2, recording-reproduction layer8 has a film thickness of 800 Å(tolerable error ±10 Å). The remaininglayers have a film thickness identical to the film thickness ofcorresponding layers of FIG. 1.

Another example of a stacked layer structure of a magneto-opticalrecording medium used in the embodiment of the present invention will bedescribed with reference to FIG. 3.

Referring to FIG. 3, the magneto-optical recording medium includes aglass substrate 1, an underlying layer 60 formed of Al deposited onglass substrate 1, a reproduction layer 61 formed of PdCo deposited onunderlying layer 60, a recording layer 4 formed of TbFeCo deposited onreproduction layer 61, and a protection layer 5 formed of SiN depositedon recording layer 4.

In the magneto-optical recording medium shown in FIG. 3, informationrecorded at recording layer 4 is transferred to reproduction layer 61 bythe exchange-coupling force generated by emission of a laser beam andreproduced at the time of reproduction. The PdCo forming reproductionlayer 61 has the property of being converted into a magnetic thin filmwith intra-plane magnetization or with perpendicular magnetizationaccording to the stress applied to the layer. The stress applied to thelayer includes tensile stress and compressive stress. The stress appliedon reproduction layer 61 is determined according to the level of thethermal expansion coefficients of the PdCo forming reproduction layer 61and underlying layer 60.

The thermal expansion coefficient of glass is approximately 0.5×10⁻⁶(1/° C.), which is extremely smaller than the thermal expansioncoefficient of PdCo which is approximately 12×10⁻⁶ (1/° C.). When PdCois directly deposited on glass substrate 1, tensile stress is exerted atPdCo when the temperature is reduced from the temperature of depositionto room temperature. In the example shown in FIG. 3, since Al layer 60(thermal coefficient is 24×10⁻⁶ (1/° C.)) is formed between the glasssubstrate 1 and the PdCo layer 61, the difference in thermal expansioncoefficient becomes larger, resulting in greater tensile stress. By thistensile stress and by the magnetic strain coefficient of PdCo being ashigh as 3.5×10⁻⁵, perpendicular magnetization anisotropy is exhibited atPdCo, so that reproduction layer 61 becomes a magnetic thin film withperpendicular magnetization at room temperature. By increasing thetemperature of the PdCo which is a magnetic thin film with perpendicularmagnetization at room temperature to a level higher than the temperatureof deposition, the PdCo is rendered to a magnetic thin film withintra-plane magnetization to allow reproduction from reproduction layer61.

An example of a stacked layer structure of a phase transition recordingmedium used in the embodiment of the present invention will be describedwith reference to FIG. 4.

Referring to FIG. 4, this phase transition recording medium includes asubstrate 1 formed of transmissive polycarbonate, a protection film 62formed of ZnS-SiO₂ on substrate 1, a recording layer 63 formed ofGe₂Sb₂Te₅ on protection film 62, a protection film 64 formed of ZnS-SiO₂on recording layer 63, a reflection film 65 formed of Al9 ₉₅Ti₅ onprotection film 64, and an ultraviolet curing resin 7 formed onreflection film 65.

Protection film 62 has a film thickness of 200 nm (tolerable error ±10nm). Recording layer 63 has a film thickness of 20 nm (tolerable error±5 nm). Protection film 64 has a film thickness of 15 nm (tolerableerror ±5 nm)). Reflection film 65 has a film thickness of 100 nm(tolerable error ±10 nm).

The phase transition recording medium has information recorded by thephase transition of a recording layer of the medium taking intoconsideration difference in the reflectance of the laser beam between anamorphous state and a crystalline state.

As to each of the stacked layers, first and second dielectric layers 2and 5, reproduction layers 3 and 61, recording layers 4 and 63,recording-reproduction layer 8, and heat radiating layer 6 are formed byRF magnetron sputtering.

The entire structure of an information recording and reproductionapparatus for a magneto-optical recording medium or a phase transitionrecording medium according to a first embodiment of the presentinvention will be described hereinafter with reference to FIG. 5.Description is provided for recording and reproducing to and from amagneto-optical disk as a magneto-optical recording medium in thefollowing embodiments.

Referring to FIG. 5, the information recording and reproductionapparatus includes a signal modulation circuit 32, a timing pulsegeneration circuit 33, a magnetic head drive circuit 34, a laser drivecircuit 35, an optical head 36, a magnetic head 37, a spindle motor 38,a servo circuit 39, a reproduction signal amplify circuit 40, a low passcircuit 41, a clock generation circuit 42, a decoder 43, a firstsynchronizing signal generation circuit 44, and a duty correctioncircuit 45.

A recording operation of the information recording and reproductionapparatus of FIG. 5 will first be described. Data representinginformation to be recorded is applied to signal modulation circuit 32 tobe modulated according to a 1-7RLL method, for example. The modulateddata is provided to timing pulse generation circuit 33 to be modifiedinto a pulse signal having a predetermined duty ratio and to have apredetermined phase difference set. The pulse signal is applied tomagnetic head drive circuit 34 and laser drive circuit 35.

Laser drive circuit 35 responds to the applied pulse signal to drive asemiconductor laser (not shown) in optical head 36. A laser beam isemitted on a magneto-optical disk 31 which is a magneto-opticalrecording medium shown in FIGS. 1-3, for example. Magnetic head drivecircuit 34 also responds to the applied pulse signal to drive magnetichead 37, whereby a record signal is recorded on magneto-optical disk 31.The relationship between the magnetic field applied to themagneto-optical disk and the laser beam is as shown in FIGS. 6A and 6B.More specifically, the magnetic field applied by magnetic head 37 isinverted in a pulsive manner between the south pole and the north pole.Since there is a transition region at the time of inversion, control isprovided to suppress optical head 36 from emitting a laser beam duringthe period corresponding to the transition region.

It is to be noted that information to be recorded is not limited to animage signal, and may be any signal such as an audio signal or a datasignal.

A reproduction operation of the information recording and reproductionapparatus of FIG. 5 will be described hereinafter.

A laser beam emitted from a semiconductor laser (not shown) in opticalhead 36 passes through an objective lens (not shown) in optical head 36to be emitted on a recording plane of magneto-optical disk 31. Reflectedlight from the recording plane is detected by a photodetector (notshown) in optical head 36. As a result, a reproduction signal andvarious error signals are obtained from optical head 36.

The reproduction signal and error signals obtained from optical head 36are provided to reproduction signal amplify circuit 40 to be amplified.The amplified reproduction signal is provided to low pass circuit 41,and the amplified error signals are applied to servo circuit 39. Lowpass filter 41 integrates the applied reproduction signal to provide thesame to decoder 43 and clock generation circuit 42. Clock generationcircuit 42 extract a clock signal from the applied reproduction signal.That clock signal is applied to servo circuit 39, decoder 43, andsynchronizing signal generation circuit 44 as an internal clock signal.

Servo circuit 39 rotates spindle motor 38 at a predetermined speed ofrotation, and also controls the objective lens in optical head 36 toeffect tracking servo control and focus servo control according to errorsignals from reproduction signal amplify circuit 40 and a clock signalfrom clock generation circuit 42. Decoder 43 decodes the reproductionsignal from low pass circuit 41 under the 1-7 method in synchronizationwith the clock signal generated by clock generation circuit 42 to outputthe same as reproduced data.

Synchronizing signal generation circuit 44 generates a synchronizingsignal to emit a pulsed laser beam according to a clock signal providedfrom clock generation circuit 42. The generated synchronizing signal isapplied to duty correction circuit 45. In response, duty correctioncircuit 45 generates a pulse signal with a predetermined duty ratio,which is provided to laser drive circuit 35. Laser drive circuit 35provides control of optical head 36 according to the applied pulsesignal to emit a pulsed laser beam in reproduction. The specific meansfor providing a pulsed laser beam will be described afterwards.

The present invention is characterized in carrying out informationreproduction at high density by emitting a pulsed laser beam from anoptical head at the time of reproduction. FIGS. 7A-7C and FIGS. 8A-8Dshow respective reproduction signal waveforms when information isreproduced from the same record domain using a continuous laser beam anda pulsed laser beam. The pulsed reproduction waveforms of FIG. 8Crepresent waveform of the output of reproduction signal amplify circuit40. As seen from FIG. 8D, the recorded information can be sufficientlyreproduced even when such pulse laser beam is used for reproduction.

A modification of the first embodiment of the present invention will bedescribed with reference to FIG. 9. The information recording andreproduction apparatus of FIG. 9 has a structure similar to that of thefirst embodiment shown in FIG. 5 except for the following points.Description of common elements will not be repeated.

The information recording and reproduction apparatus of FIG. 9 has thereproduced signal from reproduction signal amplify circuit 40 applied toa low pass circuit 41 and also to an A/D converter 49. The reproductionsignal integrated at A/D converter 49 is applied to the next-stagedecoder 43 to be demodulated.

The reproduction signal applied to low pass circuit 41 is integratedsimilarly as in the first embodiment shown in FIG. 5 to be applied toclock generation circuit 42. The output of clock generation circuit 42is applied to first synchronizing signal generation circuit 44 as in thefirst embodiment of FIG. 5. It is to be noted that in the modificationof FIG. 9, the output of first synchronizing signal generation circuit44 is applied, not only to duty correction circuit 45, but also to asecond synchronizing signal generation circuit 48. Second synchronizingsignal generation circuit 48 delays an applied first synchronizingsignal for a predetermined time period to generate a secondsynchronizing signal. The second synchronizing signal is applied to A/Dconverter 49. Similar to the embodiment of FIG. 5, duty correctioncircuit 45 generates and provides to laser drive circuit 35 a pulsesignal having a predetermined duty according to the first synchronizingsignal, whereby a pulsed laser beam is provided in reproduction.

A/D converter 49 detects and integrates a reproduction signal providedfrom reproduction signal amplify circuit 40 in synchronization with asecond synchronizing signal applied from second synchronizing signalgeneration circuit 48. FIGS. 10A-10C shows pulsed reproduction waveformobtained from reproduction signal amplify circuit 40 and sample waveformthat is held by A/D converter 49 when information is reproduced by apulsed laser beam from a record domain identical to that of FIGS. 7A and8A. A/D converter 49 sample-holds the pulsed reproduction waveform andthen applies an integration process thereon. The integrated signal isapplied to decoder 43 of the next stage.

The function of second synchronizing signal generation circuit 48 andA/D converter 49 of the embodiment of FIG. 9 will be describedhereinafter. The embodiment of FIG. 9 shows an information recording andreproduction apparatus that is particularly effective in reproducinginformation from the magnetically induced superresolutionmagneto-optical recording medium shown in FIG. 1. When the record domainshown in FIG. 11A is reproduced by a pulsed laser beam shown in FIG.11B, a reproduction waveform thereof is shown in FIG. 11C with respectto a general magneto-optical disk and in FIG. 11D with respect to amagnetically induced superresolution disk.

In the case of a general magneto-optical disk, emission of a pulsedlaser beam on the disk causes increase in temperature at a predeterminedreproduction region on the disk to reproduce information therefrom. Thispredetermined reproduction area attaining a high temperature will notincrease over the time period of the emitted pulsed beam. Therefore, thereproduced waveform from a general magneto-optical disk shows arectangular reproduced waveform. In contrast, when a pulsed laser beamis emitted on a magnetically induced superresolution disk, the region(window) of the reproduction layer from which information is reproducedincreases over the emitted time at each pulse of the laser beam.Therefore, the reproduced waveform for each pulse becomes greater overtime.

Thus, the reproduction characteristics can be improved by detecting areproduction signal at a timing where the reproduction waveform becomesgreater and takes a constant value for each pulse in reproducinginformation from a magnetically induced superresolution disk.

For this purpose, in the information recording and reproductionapparatus of FIG. 9, first synchronizing signal generation circuit 44generates a first synchronizing signal required to provide a pulsed beamaccording to an internal clock signal applied from clock generationcircuit 42. The first synchronizing signal is applied to duty correctioncircuit 45 and second synchronizing signal generation circuit 48. Secondsynchronizing signal generation circuit 48 delays the received firstsynchronizing signal for a predetermined time period to provide to A/Dconverter 49 a second synchronizing signal for sample-holding areproduction waveform at a timing where the reproduced waveform becomesgreater to take a constant value as shown in FIG. 11D. As a result, A/Dconverter 49 can detect and integrate a reproduction waveform at atiming where the reproduction waveform is greatest. In other words, thereproduction characteristics can be improved.

FIGS. 12A-12C show reproduced waveforms from a large domain and a smalldomain recorded on a magnetically induced superresolution disk. Wheninformation is to be reproduced from a large record domain as shown inFIG. 12B, the reproduced waveform becomes greater over the beam emittedtime to take a constant value for each pulse of the laser beam. Wheninformation is to be reproduced from a small record domain, thereproduced waveform will take a maximum value for each pulse of thelaser beam. Therefore, in the embodiment shown in FIG. 9, A/D converter49 must detect a reproduced waveform at a timing where the reproducedwaveform from the smallest domain takes the maximum value. Although thereproduced waveform from the larger record domain does not necessarilyreach the maximum value at this timing, reproduction can be carried outsufficiently since the record domain is large. Second synchronizingsignal generation circuit 48 has the delay time determined so as togenerate a second synchronizing signal that allows sample-holding of areproduced pulse waveform at a timing where the reproduced waveform isgreatest.

The relationship between the CN ratio and the domain length wheninformation is reproduced using a pulsed laser beam will be describedhereinafter with reference to FIG. 13. The delay time of the secondsynchronizing signal from the timing the pulse laser is turned on by thefirst synchronizing signal is employed as the parameter. A delay time of0.25 Pw (Pw: duration time of pulse light), 0.5 Pw, and 0.75 Pw aretaken as the parameter. In a region of a small domain length, the CNratio in reproduction is at least 40 dB when the delay time is 0.5 Pw.In the region of a long domain length, the CN ratio in reproduction isat least 40 dB when the delay time is in the range of 0.25 Pw˜0.75 Pw.Therefore, detection of a reproduced pulse waveform is effected at thetiming of a second synchronizing signal that is a delayed version of thefirst synchronizing signal by 0.25˜0.75 times the pulse light durationtime Pw.

According to the above-described first embodiment and modificationthereof, information can be detected with reproduction characteristicsof a sufficient level even with respect to domains of a length smallerthan 0.40 μm which was the shortest domain length allowed inconventional reproduction.

An information recording and reproduction apparatus according to asecond embodiment of the present invention will be described hereinafterwith reference to FIG. 14. The entire structure of the informationrecording and reproduction apparatus of the second embodiment is similarto the entire structure of the first embodiment shown in FIG. 5. In thesecond embodiment, optical head 36 has a characteristic structure.Therefore, description of the entire structure of the informationrecording and reproduction apparatus of the second embodiment will notbe repeated. Only the structure of optical head 36 will be describedwith reference to FIG. 14. Although description is provided ofreproducing information from a magneto-optical recording medium, thesame can be applied in reproducing information from a phase transitionrecording medium.

In optical head 36 of FIG. 14, a laser beam having a wavelength of 680nm (tolerable error ±15 nm) generated from a semiconductor laser 10driven by a semiconductor laser drive circuit 9 a receiving a controlsignal from a laser drive circuit 35 of FIG. 10 is rendered parallel bya collimator lens 11. The parallel beam passes through a polarizationplane rotary unit 12, a polarizing filter 13, and a half mirror 14 toenter objective lens 15. The laser beam is focused by objective lens 15to be emitted on a recording plane 16 a through a substrate 16 of a diskwhich is an magneto-optical recording medium.

The laser beam reflected at recording plane 16 a returns to half mirror14 via substrate 16 and objective lens 15. Half of the laser beam ispassed through half mirror 14, and the remaining half of the laser beamis reflected thereat. The laser beam reflected from half mirror 14 iscollected through a Wollaston prism 17, a collection convergent lens 18and a cylindrical lens 19 to enter a photodetector 20. A reproductionsignal and error signals such as a tracking error signal and a focuserror signal are detected by photodetector 20. In the present invention,the wavelength of the laser beam is 400-800 nm, preferably 600-700 nm,further preferably 620-650 or 665-695 nm.

Polarizing filter 13 has a structure shown in FIGS. 15A and 15B. Morespecifically, a polarizing film 132 is sandwiched by the entire facingsurfaces of a pair of transparent glass 131. This polarizing filter 132has the property of transmitting only a laser beam that is polarized ina particular direction. Transparent glass 131 can be formed of anymaterial that is transparent and has superior optical characteristicssuch as resin (polycarbonate, PMMA and the like).

FIG. 16 shows the polarization characteristics of polarizing filter 13.Polarizing filter 13 has the property of transmitting only a laser beamthat is polarized in a particular direction by polarizing film 132. Inthe embodiment that will be described hereinafter, it is assumed thatpolarizing filter 13 transmits only a laser beam that has a plane ofpolarization parallel to the sheet of the drawing.

As shown in FIGS. 15A and 15B, polarization plane rotary unit 12includes a pair of transparent glass plates 123, a pair of transparentelectrodes 121, each formed at respective inner surfaces of glass plate123, and a TN (twisted nematic) type liquid crystal 122 sandwichedbetween the pair of transparent electrodes 121. Transparent electrode121 has an outer portion 121 a and an inner portion 121 b patterned sothat voltages can be applied individually to the inner portion and theouter portion.

A recording operation using the optical head shown in FIG. 14 will bedescribed hereinafter. In recording, voltages are applied by a liquidcrystal drive circuit 9 b receiving a control signal from laser drivecircuit 35 of FIG. 5 to outer portion 121 a and inner portion 121 b ofthe transparent electrode of polarization plane rotary unit 12. As aresult, the laser beam entering TN type liquid crystal 122 parallel tothe sheet of the drawing is transmitted without having its plane ofpolarization entirely rotated and enters polarizing film 132 ofpolarizing filter 13. Since polarizing film 132 transmits only a laserbeam that is polarized in the direction parallel to the sheet of thedrawing, the incident laser beam passes through polarizing filter 13without being blocked to be emitted on a recording plane 16 a of themagneto-optical disk via half mirror 14 and objective lens 15. Recordingonto the magneto-optical disk is effected by applying a pulsed magneticfield and a recording-oriented pulsed laser beam as described alreadywith reference to FIGS. 6A and 6B. The duty of the pulsed magnetic fieldat the time of recording is 50%, and the duty of the pulsed laser beamfor recording is 30%.

A reproduction operation of the optical head of FIG. 14 will bedescribed hereinafter. In reproduction, a voltage is applied only toouter portion 121 a of the transparent electrode as shown in FIG. 15B.No voltage is applied to inner portion 121 b. As a result, the incidentlaser beam polarized in a direction parallel to the sheet of the drawinghas its plane of polarization rotated 90° by TN type liquid crystal 122only at its inner portion to be polarized in a direction perpendicularto the sheet of the drawing.

The laser beam passing through TN type liquid crystal 122 has its innerportion with a plane of polarization perpendicular to the drawing sheetblocked by polarizing film 132 of polarizing filter 13, and only itsouter portion is transmitted. The laser beam transmitted throughpolarizing filter 13 passes through half mirror 14 and objective lens 15to be emitted on recording plane 16 a of the magneto-optical recordingmedium. Since the inner portion of the laser beam is blocked asdescribed above, a laser beam having a main lobe and a pair of sidelobes is emitted on recording plane 16 a by the optical superresolutionmethod.

In the second embodiment, the laser beam for reproduction is pulsed insynchronization with the first synchronizing signal generated by firstsynchronizing signal generation circuit 44, as in the previous firstembodiment. A pulsed laser beam for reproduction is provided from theoptical head of FIG. 14 as set forth in the following.

As previously described with reference to FIG. 15B, a laser beam formedof a main lobe and a pair of side lobes is emitted on a recording plane16 a of the magneto-optical disk when the voltage applied to innerportion 121 b of the transparent electrode is turned off and the voltageapplied to outer portion 121 a of the transparent electrode is turnedon. When the voltages applied to inner portion 121 b and outer portion121 a of the transparent electrode are both turned off as shown in FIG.15A, the laser beam polarizing in a direction parallel to the sheet ofthe drawing has its plane of polarization entirely rotated 90° by TNtype liquid crystal 122, whereby the laser beam is entirely blocked bypolarizing film 132 of polarizing filter 13. Therefore a pulsed laserbeam for reproduction formed of a main lobe and a pair of side lobes canbe provided by constantly turning off the voltage applied to innerportion 121 b of transparent electrode, and turning on/off the voltageapplied to outer portion 121 a in synchronization with the firstsynchronizing signal generated by first synchronizing signal generationcircuit 44.

As an alternative method of blocking the light of only the inner portionof the laser beam, the voltages applied to the inner portion 121 b andouter portion 121 a of the transparent electrode of polarization planerotary unit 12 can be turned on/off simultaneously, and providingpolarizing film 132 of polarizing filter 13 divided into an innerportion and an outer portion differing in polarization characteristicsfrom each other.

When the numerical aperture of objective lens 15 is 0.55 (tolerableerror ±0.1) and the diameter of the effective luminance flux is 4 mm,the diameter of the inner portion 121 b of the transparent electrode isselected so that the beam diameter of the main lobe according to opticalsuperresolution becomes 0.7-1.1 μm. When the diameter of the effectiveluminance flux is not 4 mm, the diameter of inner portion 121 b isdetermined in proportion to the diameter of the effective luminance fluxso that the beam diameter of the main lobe is 0.7-1.1 μm.

Although the laser beam entering polarization plane rotary unit 12 isdescribed as being polarized in a direction parallel to the sheet of thedrawing, the present invention is not limited to this direction ofpolarization. A laser beam that polarizes in a direction perpendicularto the sheet of the drawing can be used. In this case, the voltageapplied to inner portion 121 b of the transparent electrode isconstantly turned on, and the voltage applied to outer portion 121 a isturned on/off in synchronization with the first synchronizing signal atthe time of reproduction as shown in FIGS. 17A and 17B.

More specifically, as shown in FIG. 17B, turning off the voltage appliedto outer portion 121 a causes the laser beam that polarizes in adirection perpendicular to the sheet of the drawing to have the plane ofpolarization rotated 90° only at its outer portion by TN type liquidcrystal 122 to be polarized in a direction parallel to the drawingsheet. As a result, the inner portion of the laser beam is blocked, andonly the outer portion of the laser beam is transmitted throughpolarizing filter 13 by polarizing film 132 in polarizing filter 13.When the voltage applied to outer portion 121 a is turned on, the laserbeam polarizing in a direction perpendicular to the drawing sheetdirectly passes through TN type liquid crystal 122 without having itsplane of polarization entirely rotated by TN type liquid crystal 122. Asa result, the laser beam will be entirely blocked by polarizing film 132of polarizing filter 13. Therefore, by turning on/off the voltageapplied to outer portion 121 a of the transparent electrode insynchronization with the first synchronizing signal during reproduction,a pulsed laser beam of a main lobe and a pair of side lobes according tothe optical superresolution method can be provided even when a laserbeam that polarizes in a direction perpendicular to the drawing sheet isused.

When the above-described laser beam polarized in a directionperpendicular to the drawing sheet is used, it is not necessary toprovide the entire transparent electrode 121 and TN type liquid crystal122 in polarization plane rotary unit 12. More specifically, TN typeliquid crystal 122 a and transparent electrode 121 a are to be providedonly for the region corresponding to the outer portion of the laser beamthat polarizes in a direction perpendicular to the drawing sheet. Whenthe voltage applied to transparent electrode 121 a is turned off, thelaser beam that polarizes in a direction perpendicular to the sheet ofthe drawing has the plane of polarization rotated 90° only at the outerportion by TN type liquid crystal 122 a to be polarized in a directionparallel to the drawing sheet. The inner portion of the laser beam thatis polarized in a direction perpendicular to the drawing sheet isdirectly transmitted. As a result, the laser beam has its inner portionblocked and only the outer portion transmitted by polarizing film 132 ofpolarizing filter 13.

When the voltage applied to outer portion 121 a is turned on, the laserbeam polarizing in a direction perpendicular to the drawing sheet isdirectly transmitted without having its plane of polarization entirelyrotated by TN type liquid crystal 122 a. The inner portion of the laserbeam that polarizes in a direction perpendicular to the drawing sheet isdirectly transmitted. As a result, the laser beam is entirely blocked bypolarizing film 132 of polarizing filter 13. Therefore, by turningon/off the voltage applied to the outer portion of transparent electrode121 a, a pulsed laser beam formed of a main lobe and a pair of sidelobes can be obtained by the optical superresolution method.

The on/off of the voltages applied to transparent electrodes 121 a and121 b is controlled by laser drive circuit 35 in synchronization withthe first synchronizing signal applied from duty correction circuit 45of the embodiments shown in FIGS. 6 and 9.

When a laser beam polarizing in a direction perpendicular to the drawingof the sheet as described above is to be used, the diameter of innerportion 121 b of the transparent electrode (FIGS. 17A and 17B) or thediameter of the region where TN type liquid crystal 122 is not provided(FIGS. 18A and 18B) is selected so that the beam diameter of the mainlobe by optical superresolution becomes 0.7-1.1 μm for an objective lens15 having a numerical aperture of 0.55 (tolerable error ±0.1) and forthe effective luminance flux having a diameter of 4 mm. When thediameter of effective luminance flux diameter is not 4 mm, the diameterof the inner portion is determined in proportion to the effectiveluminance flux so that the beam diameter of the main lobe is 0.7-1.1 μm.

Although a polarization plane rotary unit 12 and a polarizing filter 13are used in the above embodiments, a polarization selective hologram, aguest-host element, a glass polarizer, a polarization beam splitter, andthe like can be used instead.

For example, a liquid crystal shutter as shown in FIGS. 19A and 19B canbe used instead of polarization plane rotary unit 12 and polarizingfilter 13. The liquid crystal shutter includes a pair of transparentglass plates 21, a transparent electrode 22 formed at respective innersurfaces of the pair of glass plates 21, and a guest-host type liquidcrystal 23 sandwiched between the pair of glass plates 21. Theguest-host type liquid crystal is divided into an outer portion and aninner portion to allow voltages to be applied independently. Theguest-host type liquid crystal is an element that exhibits polarizingselective characteristics only when a voltage is applied.

When the voltage applied to the inner portion is turned on and thevoltage applied to the outer portion is turned off as shown in FIG. 19A,the laser beam polarizing in a direction perpendicular to the sheet ofthe drawing has only its outer portion transmitted. When the voltageapplied to the inner portion and the outer portion are both turned on asshown in FIG. 19B, the laser beam polarizing in a directionperpendicular to the drawing sheet is entirely blocked by the guest-hosttype liquid crystal. Thus, a pulsed laser beam formed of a main lobe anda pair of side lobes by optical superresolution can be providedaccording to such a structure.

Alternatively, a glass polarizer as shown in FIGS. 20A and 20B can beused instead of polarizing filter 13 in the above embodiment. The glasspolarizer is fabricated by arranging silver compound in a predetermineddirection in glass, as shown in FIG. 20A, and reducing the surface todeposit silver. The reduced silver film exhibits polarizationcharacteristics. Although it is desirable to use silver as the materialfor providing polarization characteristics in the glass polarizer, othermetal materials can be used as long as it provides polarizationcharacteristics.

Although TN type liquid crystal 122 is used as polarization plane rotaryunit 12 in the above embodiment, a STN (Super Twisted Nematic) liquidcrystal or a ferroelectric type liquid crystal can be used. When apositive voltage is applied for short time, the ferroelectric typeliquid crystal causes the plane of polarization of the laser beam to berotated 45° and maintains that state. When a negative voltage is appliedfor a short time period, the ferroelectric type liquid crystal causesthe plane of polarization of the laser beam to be rotated 45° in adirection opposite to that of the positive voltage application, andmaintains that state. Therefore, there will be a difference of 90° inthe direction of polarization of the transmitted laser beam betweenapplication of a positive voltage and a negative voltage. Therefore, byrotating the direction of polarization of the incident laser beam 45° inadvance than the direction of polarization of the above-describedembodiment, the plane of polarization of the laser beam can be rotated90°, as in the above embodiment. Usage of such a ferroelectric typeliquid crystal provides the advantage of reducing power consumptionsince the voltage is to be applied only for a short time periodinitially.

Furthermore, a Pockels cell as shown in FIG. 21 can be used instead ofTN type liquid crystal 122 in the above embodiment. When a predeterminedvoltage is applied, a Pockels cell polarizes the laser beam having aplane of polarization in a vertical direction in the sheet of FIG. 21into a laser beam having a plane of polarization in the horizontaldirection in the same drawing. Since the rotating angle of the plane ofpolarization can be altered by adjusting the applied voltage, therotating angle of the plane of polarization can be adjusted so as toobtain the optimum recording and reproduction characteristics.

Also, a Faraday element that rotates the plane of polarizationmagnetically as shown in FIG. 22 can be used instead of TN type liquidcrystal 122 in the above embodiment. Since the direction of passage ofthe laser beam is identical to the direction of applied magnetic field Hin a Faraday element, the plane of polarization can be rotated bywinding a coil around a tube that supports the Faraday element.Accordingly, assembly and the structure of a Faraday element is simple.

The configuration of the center portion of the laser beam that blockslight does not necessarily have to be circular. The center portion ofthe laser beam may have a polygonal configuration of any of a triangleto an octagon.

Thus, according to the second embodiment of the present invention, apulsed laser beam by optical superresolution can be provided only at thetime of reproduction using a single optical system to reproduceinformation at high density from a magnetically induced superresolutionmagneto-optical disk.

An information recording and reproduction apparatus according to a thirdembodiment of the present invention will be described hereinafter withreference to FIG. 23. The information recording and reproductionapparatus of the third embodiment has a structure similar to that of theinformation recording and reproduction apparatus of the first embodimentshown in FIG. 5. The information recording and reproduction apparatus ofthe third embodiment is characteristic in its optical head 36.Therefore, only the optical head of the information recording andreproduction apparatus of the third embodiment will be described.

The structure of the optical head of FIG. 23 is similar to the opticalhead of the second embodiment shown in FIG. 14 except for the structureof polarization plane rotary unit 12′. Therefore, description of commonelements will not be repeated.

In contrast to the optical head of the second embodiment shown in FIG.14 in which the laser beam has the inner portion blocked and formed byoptical superresolution to provide a pulsed laser beam, the thirdembodiment shown in FIG. 23 provides a pulsed laser beam without usingthe optical superresolution method.

In optical head 36 of FIG. 23, a laser beam having a wavelength of 680nm (tolerable error ±15 nm) generated from a semiconductor laser 10 ismade parallel by a collimator lens 11 to enter an objective lens 15through a polarization plane rotary unit 12′, or polarizing filter 13,and a half mirror 14. The laser beam is focused by objective lens 15 tobe emitted on recording plane 16 a through a substrate of disk 16 whichis a magneto-optical recording medium.

The laser beam reflected at recording plane 16 a returns to half mirror14 via substrate 16 and objective lens 15. Half of the laser beam passesthrough half mirror 14 and the remaining half is reflected thereat. Thelaser beam reflected from half mirror 14 passes through Wollaston prism17, collection lens 18, and cylindrical lens 19 to be converged to enterphotodetector 20. A reproduction signal and error signals are detectedby photodetector 20. In the third embodiment of the present invention,the wavelength of the laser beam is 400-800 nm, preferably 600-700 nm,and further preferably 620-650 or 665-695 nm.

Polarizing filter 13 has a structure identical to that of the secondembodiment. As shown in FIGS. 24A and 24B, polarizing filter 13 includesa pair of transparent glass plates 131, and a polarizing film 132sandwiched by the pair of glass plates 131. Polarizing film 132 has theproperty of transmitting only a laser beam that is polarized in aparticular direction. Any material that is transparent and has superioroptical characteristics can be used for transparent glass 131. Forexample, resin (polycarbonate PMMA, and the like) can be used.

The polarizing characteristics of polarizing filter 13 is as shown inFIG. 16, similar to that of the second embodiment. More specifically,polarizing filter 13 transmits only a laser beam that polarizes in aparticular direction by means of polarizing film 132. In the presentthird embodiment, it is assumed that a laser beam having a plane ofpolarization parallel to the drawing sheet is transmitted.

Polarization plane rotary unit 12′ includes a pair of transparent glassplates 123, a pair of transparent electrodes 121 formed on therespective inside surfaces of glass plates 123, and a TN type liquidcrystal 122 sandwiched between the pair of transparent electrodes 121.

A recording operation of the optical head of the third embodiment willbe described hereinafter. In recording, a voltage is applied entirely totransparent electrode 121 of polarization plane rotary unit 12′ as shownin FIG. 24B. The laser beam entering TN type liquid crystal 122polarizing in a direction parallel to the drawing sheet is passedthrough without having its plane of polarization entirely rotated. Then,the laser beam enters polarizing film 132 of polarizing filter 13. Sincepolarizing film 132 transmits only a laser beam that is polarized in adirection parallel to the drawing sheet, the incident laser beam passesthrough polarizing filter 13 without being entirely blocked to beemitted on recording plane 16a of the magneto-optical disk via halfmirror Ki and objective lens 15. Recording onto a magneto-optical diskis carried out by a pulsed magnetic field and a pulsed laser beam, as inthe second embodiment. The pulsed magnetic field has a duty of 50%, andthe pulsed laser has a duty of 30%.

A reproduction operation of the optical head of the third embodimentwill be described hereinafter. In reproduction, the voltage applied totransparent electrode 121 of polarization plane rotary unit 12′ isturned on/off in synchronization with the first synchronizing signalgenerated from first synchronizing signal generation circuit 44 of theinformation recording and reproduction apparatus shown in FIGS. 5 or 9.As a result, the incident laser beam polarized in a direction parallelto the drawing sheet passes through TN liquid crystal 122 without havingits plane of polarization entirely rotated to enter polarizing filter 13when the voltage applied to transparent electrode 121 is turned on.Since polarizing film 132 in polarizing filter 13 passes through onlythe laser beam that is polarized in a direction parallel to the drawingsheet as described above, the laser beam is transmitted entirely throughpolarizing filter 13.

When the voltage applied to transparent electrode 121 is turned off, thelaser beam that is polarized in a direction parallel to the drawingsheet has its plane of polarization entirely rotated 90° by TN typeliquid crystal 122 so as to polarize in a direction perpendicular to thedrawing sheet. As a result, the incident laser beam in polarizing filter13 is entirely blocked by polarizing film 132. Therefore, a pulsed laserbeam can be provided for reproduction by turning on/off the voltageapplied to transparent electrode 121.

Although polarizing filter 13 is used in the third embodiment, apolarizing selective hologram, a guest-host element, a glass polarizer,a polarization beam splitter and the like can be used instead, as in thecase of the second embodiment.

In the third embodiment, TN type liquid crystal 122 is used aspolarization plane rotary unit 12′. The present invention is not limitedto this, and a STN type liquid crystal, a ferroelectric type liquidcrystal, and the like can be used instead, as in the second embodiment.Also, the plane of polarization can be rotated electrically, not by aliquid crystal, but by using a Pockels cell. Furthermore, a Faradayelement can be used to rotate the plane of polarization magnetically.

In the second and third embodiments, a pulsed laser beam forreproduction is provided by repeating blocking/transmission of the laserbeam in synchronization with a pulse signal generated by duty correctioncircuit 45 shown in FIGS. 5 or 9. A pulsed laser beam can be obtained,not only by this method, but by turning on/off semiconductor laser 10per se in optical head 36 in synchronization with a pulse signalgenerated by duty correction circuit 45 of FIGS. 5 and 9.

Furthermore, the method of repeating blocking/transmission of a laserbeam is not limited to the method described in the second and thirdembodiments in which a combination of a polarization plane rotary unitand a polarizing filter is used. The laser beam can repeatedly beblocked/transmitted by inserting and withdrawing a light blocking bodyinto and from an optical path by mechanical means.

According to the second and third embodiments, information can bereduced from a record domain having the shortest length of 0.15-0.30 μm.

FIG. 25 shows the relationship of jitter over laser power at the time ofreproduction according to the second and third embodiments.

In FIG. 25, the duty values of a pulsed laser beam of 35%, 40%, 50% and100% is employed as parameter. It is appreciated from the graph of FIG.25 that a range of 1.0-2.5 mW is appropriate for the laser power at thetime of reproduction. It is also appreciated that the jitter inreproduction using a pulsed laser beam is smaller than the case using acontinuously emitted light of 100% in duty. It is understood that thetime period of a laser beam emission becomes shorter as the duty whichindicates the degree of a pulsed laser beam becomes smaller to result ina smaller jitter during reproduction. Thus, favorable reproductioncharacteristics can be exhibited as the degree of the pulsed laser beamis increased. In the above-described embodiments, the reproductionresolution is improved by inserting the light blocking body into thedirection of the track of the disk to improve the line density withoutbeing affected by side lobes when the optical superresolution method isemployed. The track density can also be improved without being affectedby side lobes by inserting the light blocking body into the tangentialdirection at the time of recording. By setting a different direction forthe light blocking body to be inserted between recording andreproduction, the recording density can further be improved.

A fourth embodiment of the present invention relates to an informationrecording and reproduction apparatus of a recording medium in which awobble is provided at the wall of a groove of a recording medium.

A recording medium such as a conventional magneto-optical disk is knownto have a guide groove formed at the surface for tracking. Formation ofa wobble at a relatively long predetermined cycle on at least onesidewall of the guide groove has been proposed. The wobble isimplemented by forming guide groove so that the plane configuration ofat least one sidewall of the guide groove has a gentle sine waveformmodulated by an address information signal, a synchronizing signal, andthe like. FIG. 26 is a schematic diagram showing an example of the planeconfiguration of such a wobble. In this example, a wobble is provided atboth sidewalls of a groove. Referring to FIG. 26, domains 272 eachhaving a length corresponding to each record signal is formed in thegroove or land where wobble 271 is formed. Recording onto the groove orland is effected by applying a pulsed magnetic field and a pulsed laserbeam as described in the first embodiment. The predetermined cycle of awobble is in the range of 0.8-35 μm.

FIG. 27 is a block diagram showing an entire structure of an informationrecording and reproduction apparatus according to the fourth embodimentof the present invention. This apparatus is suitable to reproduceinformation from a recording medium in which a wobble is formed as shownin FIG. 26. The information recording and reproduction apparatus of FIG.27 is similar to the information recording and reproduction apparatus ofthe first embodiment shown in FIG. 5 except for the elements describedin the following. Description of common elements will not be repeated.

Specifically, the information recording and reproduction apparatus ofthe fourth embodiment includes an external synchronizingsignal,generation circuit 46 to receive a wobble signal output from areproduction signal amplify circuit 40 for generating an externalsynchronizing signal, and a synchronizing signal input circuit 47 toreceive the external synchronizing signal and an internal clock fromclock generation circuit 42 for generating synchronizing signal, insteadof first synchronizing signal generation circuit 44 and duty correctioncircuit 45 of the first embodiment shown in FIG. 5. The output ofsynchronizing signal input circuit 47 is provided to laser drive circuit35.

FIG. 28 is a schematic diagram for describing the principle of detectinga reproduction signal, a wobble signal, and error signals by thephotodetector in optical head 36 of FIG. 27. Referring to FIG. 28, aphotodetector 290 is divided into six regions, A, B, C, D, E and F.Photodetector 290 is arranged as shown in FIG. 28 with respect to thetracking direction shown by arrow 297 and the track direction shown byarrow 298.

As previously described with reference to FIGS. 14 and 23, reflectedlight of the laser beam from the signal recording plane passes through aWollaston prism to be divided into three beams. Among these three beams,the beams at both sides are detected by regions E295 and F296,respectively, to be detected as a reproduction signal [DE−DF] which isthe difference between the intensities DE and DF of the detected laserbeams.

The middle beam out of the three beams is detected by regions A291,B292, C293 and D294 to have [DA+DB]−[DC+DD] detected as a superimposedsignal of a wobble signal and a tracking error signal, and[DA+DC]+[DB+DD] detected as a focus error signal, which are calculatedfrom the intensities DA, DB, DC, and DD of the laser beam detected atrespective regions.

FIG. 29A shows the waveform of the above-mentioned signal which is asuperimposition of a wobble signal and a tracking error signal. Thesignal of FIG. 29A detected by photodetector 290 of optical head 36 isdivided into a wobble signal of high frequency and a tracking errorsignal of low frequency by reproduction signal amplify circuit 40. Thewobble signal is applied to external synchronizing signal generationcircuit 46, and the tracking error signal is applied to servo circuit39.

Referring to FIG. 27 again, the above-described reproduction signal[DE-DF] obtained from optical head 36 is amplified by reproductionsignal amplify circuit 40 to be provided to low pass circuit 41. Lowpass circuit 41 integrates the applied signal to provide the integratedsignal to decoder 43 and clock generation circuit 42. The clockcomponent is extracted from the reproduction signal by clock generationcircuit 42 to be applied to servo circuit 39, synchronizing signal inputcircuit 47, and decoder 43.

The reproduction signal integrated by low pass circuit 41 is demodulatedby the 1-7 method in synchronization with the clock signal from clockcircuit 42 by decoder 43 to be output as reproduced data. Servo circuit39 rotates spindle motor 38 at a predetermined speed of rotation andcontrols objective lens 15 in optical head 36 to carry out control ofthe tracking servo and the focus servo according to the tracking errorsignal and the focus error signal from reproduction signal amplifycircuit 40 and the clock signal from clock generation circuit 42.

The operation of external synchronizing signal generation circuit 46will be described with reference to FIGS. 29A-29D. A wobble signal ofhigh frequency separated from the waveform of FIG. 29A by reproductionsignal amplify circuit 40 is applied to external synchronizing signalgeneration circuit 46. FIG. 29B shows this wobble signal enlarged in thedirection of the time axis. External synchronizing signal generationcircuit 46 binarizes this wobble signal as shown in FIG. 29C to generatea synchronizing signal that is in synchronization with the rising timingof this binarized signal (FIG. 29D). More specifically, externalsynchronizing signal generation circuit 46 functions to generate asynchronizing signal in synchronization with the wobble which is areference signal provided at the sidewall of the groove shown in FIG.26, i.e. an external synchronizing signal. The generated externalsynchronizing signal is applied to the next-stage synchronizing signalinput circuit 47.

The specific structure of external synchronizing signal generationcircuit 46 will be described with reference to FIG. 30. The wobblesignal (FIG. 29B) from reproduction signal amplify circuit 40 isbinarized by a comparator 461. Since a wobble signal is generally asignal modulated by a predetermined reference information signal, thebinarized wobble signal is demodulated by demodulator 462. Thedemodulated binarization wobble signal (FIG. 29C) and a predeterminedreference pulse signal from a frequency divider 466 are applied to aphase comparator 463. Phase comparator 463 generates a plus error signaland a minus error signal when the phase of the predetermined referencepulse signal is ahead and behind, respectively, to provide a phase errorsignal formed of this plus error signal or minus error signal to a lowpass filter (LPF) 464. LPF 464 averages for a predetermined time theplus or minus error signal provided from phase comparator 463 to apply aresultant plus or minus voltage signal to a voltage control oscillationcircuit (VCO) 465. VCO 465 responds to the plus or minus voltage valuefrom LPF 464 to generate and provide to frequency divider 466 afrequency signal for altering the phase of the predetermined referencepulse signal. Frequency divider 466 alters the phase of thepredetermined reference pulse signal according to the frequency from VCO465. The predetermined reference pulse signal having its phase alteredis applied to one input of phase comparator 463.

Also, the output of VCO 465 is applied to synchronizing signal inputcircuit 47 as an external synchronizing signal (FIG. 29D).

FIGS. 31A-31F are timing charts for describing in detail the operationof the external synchronizing signal generation circuit shown in FIG.30. Reproduction signal amplify circuit 40 applies the wobble signalshown in FIG. 31A to the input of comparator 461. FIG. 31B shows thiswobble signal partially enlarged in the direction of the time axis.Comparator 461 generates a binarized version of this wobble signal (FIG.31C). The binarized signal is applied to demodulator 462 to be subjectedto a demodulation process required for a wobble signal. The demodulatedbinarized wobble signal is applied to an input of phase comparator 463.Since this binarized wobble signal has its phase fluctuated as shown inFIG. 31C, it can not be directly used as a reference signal forgenerating a proper synchronizing signal. Phase comparator 463 isprovided for the purpose of removing the fluctuation in the phase ofthis wobble signal to be set as the correct reference of synchronizingsignal generation. Phase comparator 463 compares the phases of thiswobble signal and a reference pulse signal (FIG. 31D).

Phase comparator 463 generates a pulse error signal and a minus errorsignal when the phase of the predetermined reference pulse signal isahead and behind, respectively, of the binarized wobble signal. Thegenerated signal is provided to LPF 464 as a phase error signal shown inFIG. 31E.

LPF 464 averages the applied phase error signal within a predeterminedtime to identify whether the phase error signal is eventually a plusvalue or a minus value. A phase error signal taking a plus valueindicates that the phase of the predetermined reference pulse signal isahead of the binarized wobble signal. A minus value indicates that thephase of the predetermined reference pulse signal is behind thebinarized wobble signal. LPF 464 provides this resultant voltage signalto VCO 465. VCO 465 generates a signal of a frequency to delay oradvance the phase of the predetermined reference pulse signal accordingto the applied plus or minus voltage signal. Frequency divider 466delays or advances the phase of the predetermined reference pulse signalaccording to the signal frequency from VCO 465 to provide the alteredreference signal to one input of phase comparator 463. This operation iscarried out until the phase of the binarized wobble signal matches thatof the predetermined reference pulse signal. A predetermined referencepulse signal obtained when the phases of both signals match is shown inFIG. 31F. After the phases are made to match each other, the referencepulse signal from VCO 465 is output as an external synchronizing signal,and also applied to synchronizing signal input circuit 47 of thesubsequent stage.

The demodulation operation by demodulator 462 is not necessary when thewobble at the sidewall of the groove is provided simply at a constantcycle, and not demodulated by a predetermined reference informationsignal.

Referring to FIG. 27 again, synchronizing signal input circuit 47receives the above-described external synchronizing signal generated byexternal synchronizing signal generation circuit 46 and an internalclock in synchronization with the reproduction signal, generated byclock generation circuit 42.

The operation of synchronizing signal input circuit 47 will be describedwith reference to FIGS. 32A-32C. FIG. 32A shows an externalsynchronizing signal generated by external synchronizing signalgeneration circuit 46, which corresponds to FIG. 31D. FIG. 32B shows aninternal clock separated from a reproduction signal by clock generationcircuit 42. Since the phase of external synchronizing signal 310 shownin FIG. 32A is generally deviated from the phase of internal clocksignal 311 shown in FIG. 32B, synchronizing signal input circuit 47carries out correction so that the phase of external synchronizingsignal 310 matches that of internal clock signal 311 to generate asynchronizing signal 312 (FIG. 32C) for emitting a pulsed laser beam toa magneto-optical disk in synchronization with a reproduction signal.

The reason why an external synchronizing signal of the presentembodiment is required will be described hereinafter with reference toFIGS. 33A-33C. Essentially, a synchronizing signal for emitting a laserbeam must be generated in synchronization with an internal clock as inthe first embodiment of FIG. 5 for the purpose of providing emission ofa pulsed laser beam in synchronization with a reproduction signal.However, when there is a missing portion in the reproduction signal fromthe recording medium as shown in FIG. 33B, internal clock 313 will alsoinclude a missing portion to bar generation of a synchronizing signalrequired to provide a pulsed laser beam for reproduction.

In the fourth embodiment, an external synchronizing signal is generatedaccording to a wobble that is previously formed on a medium to have thephase of that external synchronizing signal match the phase of theinternal clock. Therefore, a synchronizing signal 321 for providing apulsed laser beam can reliably be generated even if the internal clockitself is missing (FIG. 33C). It is generally possible to generate asynchronizing signal in synchronization with a reproduction signal byusing a PLL circuit when a reproduction signal is partially missing.However, a synchronizing signal cannot be generated with a conventionalPLL circuit if a great amount of data such as that corresponding to onetrack is missing. The fourth embodiment is particularly effective insuch a case by generating a synchronizing signal for providing a pulsedlaser beam according to an external synchronizing signal generated basedon the wobble of the recording medium.

Referring to FIG. 27 again, a synchronizing signal for providing apulsed laser beam is applied to laser drive circuit 35 fromsynchronizing signal input circuit 47. Laser drive circuit 35 providescontrol of optical head 36 according to the applied synchronizing signalto provide a pulsed laser beam for reproduction.

A first modification of the fourth embodiment of the present inventionwill be described with reference to FIG. 34. The modification of FIG. 34is similar to the fourth embodiment shown in FIG. 27 except for thefollowing points. Description of common elements will not be repeated.In the first modification of FIG. 34, a duty correction circuit 45 isinserted between synchronizing signal input circuit 47 and laser drivecircuit 35. Duty correction circuit 45 receives the synchronizing signalthat is required to provide a pulsed laser beam, generated atsynchronizing signal input circuit 47. Duty correction circuit 45generates a pulse signal of a predetermined duty, which is provided tolaser drive circuit 35. Laser drive circuit 35 controls optical head 36to provide a pulsed laser beam for reproduction according to the appliedpulse signal.

A second modification of the fourth embodiment of the present inventionwill be described with reference to FIG. 35. The second modification ofFIG. 35 is similar to the first modification of FIG. 34 except for thefollowing points. Description of common elements will not be repeated.

Referring to FIG. 35, a reproduction signal determination circuit 50 isprovided to determine the amplitude of a reproduction signal from lowpass circuit 41 to provide the determined result to duty correctioncircuit 45. The operation of reproduction signal determination circuit50 will be described hereinafter.

Reproduction signal determination circuit 50 determines whether thepeak-peak value of a reproduction signal from low pass circuit 41arrives at a predetermined value. For example, when the signal ismodulated by the 1-7 method and recorded, determination is made whetherthe ratio of the intensity of the 8T signal which is the longest signalto the intensity of the 2T signal which is the shortest signal, i.e.2T/8T, is within the range of 0.1-0.7, preferably in the range of0.3-0.5. Reproduction signal determination circuit 50 provides thedetermined result to duty correction circuit 45 to adjust the duty of apulse signal generated from duty correction circuit 45 so that the ONtime of the laser beam is increased when 2T/8T takes a value outside andsmaller than the above-described range, and to reduce the ON time of thelaser beam when 2T/8T is outside and greater than the above range.Although the duty of the pulse laser beam is adjusted according to thelevel of the amplitude of a reproduction signal in the example of FIG.35, a structure may be provided so that the intensity of the laser beamis adjusted instead.

Duty correction circuit 45 determines the duty in generating a pulsesignal according to the determination result from reproduction signaldetermination circuit 50 to generate a pulse signal from thesynchronizing signal output from synchronizing signal input circuit 47according to the determined duty. The generated pulse signal is providedto laser drive circuit 35. According to the modification of FIG. 35, theduty of a laser beam can be corrected according to the intensity of thereproduced signal by provision of reproduction signal determinationcircuit 50.

A third modification of the fourth embodiment of the present inventionwill be described with reference to FIG. 36. The modification of FIG. 36is similar to the fourth embodiment shown in FIG. 27 except for thefollowing points. Description of common elements will not be repeated.

In the third modification, a second synchronizing signal generationcircuit 48 is provided for generating and supplying to decoder 43 asecond synchronizing signal according to a synchronizing signal toprovide a pulsed laser beam from synchronizing signal input circuit 47.The operation of second synchronizing signal generation circuit 48 willbe described with reference to FIGS. 37A and 37B. FIG. 37A shows thesynchronizing signal generated by synchronizing signal input circuit 47(312 of FIG. 32C) partially enlarged in the direction of the time axis.Second synchronizing signal generation circuit 48 delays synchronizingsignal 312 from synchronizing signal input circuit 47 by a predeterminedtime of τ to generate a second synchronizing signal (314 of FIG. 37B).This second synchronizing signal 314 is applied to decoder 43. Secondsynchronizing signal 314 generated by second synchronizing signalgeneration circuit 48 is applied to decoder 43 as a synchronizing signalfor detecting a reproduction signal at the time point when thereproduction signal from the magneto-optical recording medium ismaximum, similar to second synchronizing signal generation circuit 48included in the modification of the first embodiment of FIG. 9.According to second synchronizing signal 314 from second synchronizingsignal generation circuit 48, decoder 43 detects a reproduction signalfrom low pass circuit 41 to apply a decode process.

Since a second synchronizing signal for detecting a reproduction signalis generated according to an external synchronizing signal formedaccording to a wobble of the recording medium in the modification shownin FIG. 36, the second synchronizing signal can be continuouslygenerated for determining the detection timing of a reproduction signaleven when there is a missing portion in the internal clock due to a dropin a reproduction signal from the recording medium. A reproductionsignal can be reliably detected irrespective of whether there is amissing portion in the internal clock. In the present modification, thevalue of τ is within the range of −180°˜+180° corresponding to theamount of delay between the first synchronizing signal and the secondsynchronizing signal previously described with reference to FIG. 9.

A fourth modification of the fourth embodiment of the present inventionwill be described with reference to FIG. 38. The modification of FIG. 38is similar to the modification of FIG. 36 except for the followingpoints. Description of common elements will not be repeated.

In the modification of FIG. 38, a duty correction circuit 45 is providedbetween synchronizing signal input circuit 47 and laser drive circuit35. The function of duty correction circuit 45 has already beendescribed in association with the first modification shown in FIG. 35.Description thereof will not be repeated.

A fifth modification of the fourth embodiment of the present inventionwill be described with reference to FIG. 39. The modification of FIG. 39is similar to the modification of FIG. 38 except for the followingpoints. Description of common elements will not be repeated.

In the modification of FIG. 39, a reproduction signal determinationcircuit 50 is provided between low pass circuit 41 and duty correctioncircuit 45. More specifically, the modification of FIG. 39 is acombination of the modifications of FIGS. 38 and 35. Secondsynchronizing signal generation circuit 48 and reproduction signaldetermination circuit 50 have functions described in association withthe respective modifications of FIGS. 36 and 35. Description ofrespective circuits will not be repeated. According to the structureshown in FIG. 39, the following advantage is obtained. In themodification of FIG. 39, the duty of the synchronizing signal forproviding a pulsed laser beam can be corrected and a reproduction signalcan be detected at the time point when the reproduction signal isgreatest according to the intensity of the signal actually reproducedfrom the recording medium. Therefore, the reproduction characteristicscan further be improved.

A sixth modification of the fourth embodiment of the present inventionwill be described with reference to FIG. 40. The modification of FIG. 40is similar to the modification of FIG. 39 except for the followingpoints. Description of common elements will not be repeated.

In the modification of the FIG. 40, an error determination circuit 51 isprovided to receive an output of decoder 43 to carry out errordetermination. Error determination circuit 51 receives reproduced datathat is decoded by decoder 43 to detect an error rate for determiningwhether a reproduction signal is detected actually at the time pointwhen the waveform of the reproduction signal is greatest. A signalindicating the determination result is applied to second synchronizingsignal generation circuit 48. Second synchronizing signal generationcircuit 48 corrects the phase of the second synchronizing signal so thatdetection of a reproduction signal is carried out when the reproductionsignal is greatest according to the determination result from errordetermination circuit 51. In the above-described fourth embodiment andmodifications thereof shown in FIGS. 27, 34, 35, 36, 38, 39 and 40, anoptical head 36 for providing a pulsed laser beam is used having thestructure of the optical head in the second embodiment of FIG. 14.Therefore, an optical superresolution method can be applied on thepulsed laser beam while providing a pulsed laser beam according to asynchronizing signal at the time of reproduction.

It is to be noted that synchronizing signal input circuit 47 can beomitted in the fourth embodiment and modifications thereof shown inFIGS. 27, 34, 35, 36, 38, 39 and 40. In this case, an externalsynchronizing signal generated from external synchronizing signalgeneration circuit 46 is directly applied to laser drive circuit 35,duty correction circuit 45, or second synchronizing signal generationcircuit 48. The phase correction of the external synchronizing signalaccording to an internal clock generated by clock generation circuit 42is not carried out. In other words, a synchronizing signal for providinga pulsed laser beam and a second synchronizing signal for defining thetime point when a reproduction signal is greatest are generated directlyaccording to an external synchronizing signal.

In the above-described fourth embodiment and modifications thereof, anexternal synchronizing signal is generated according to a wobble formedat a sidewall of a groove in a magneto-optical disk. However, anexternal synchronizing signal may be generated according to a pit formedat the land or groove of the disk.

Furthermore, a structure may be provided in which an externalsynchronizing signal is generated according to a fine clock markprovided at a sidewall of the groove of a disk. FIGS. 41A-41C areschematic diagrams showing the plan configurations of a groove in whichsuch a fine clock mark is provided. The fine clock mark is a referencemark of a relatively abrupt plane configuration formed at apredetermined interval along the groove to indicate the beginning ofdata, recording/reproduction timing of a signal, determining whether thelaser beam is on the center line of the track, and the like. The fineclock mark may be formed so as to be overlay on a wobble having arelatively gentle waveform at a predetermined interval. In the examplesshown in FIGS. 41A, 41B and 41C, a fine clock mark 420 is formed at bothsidewalls of a groove. In the example shown in FIG. 41A, a wobble is notprovided at regions other than the region where fine clock mark 420 isprovided. In the example of FIG. 41B, a wobble is provided also atregions other than the region where fine clock mark 420 is formed. Inthe example of FIG. 41C, a wobble is formed only at one sidewall of thegroove in regions other than the region where fine clock mark 420 isprovided. The advantages of the above-described fourth embodiment andmodifications thereof can be obtained even when a fine clock mark 420 isdetected instead of wobble to be used as the reference for generating anexternal synchronizing signal.

According to the fourth embodiment and modifications thereof, anexternal synchronizing signal can be generated from a referenceinformation signal such as a wobble, a fine clock mark, a pit, and thelike provided at the surface of a medium even when there is a missingportion in reproduction signal. Thus, a pulsed laser beam forreproduction can be implemented.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. An information recording and reproductionapparatus for an information recording medium, comprising: optical meansfor emitting a laser beam on a plane of said information recordingmedium in which a signal is recorded, and detecting a laser beamreflected from said signal recording plane, said optical meanscomprising a light source for generating said laser beam, polarizationdirection switching means for transmitting the generated laser beamwhile selectively rotating the direction of polarization of said laserbeam, polarization selecting means receiving the laser beam transmittedthrough said polarization direction switching means for transmittingonly the laser beam that is polarized in a particular direction, and anobjective lens subject to tracking control so as to be displaced withrespect to said signal recording plane for collecting the laser beamtransmitted through said polarization selecting means on said signalrecording plane, information reproduction means for reproducinginformation from the laser beam detected by said optical means; drivemeans for driving said optical means so as to render the emitted laserbeam into a pulsed laser beam; and first synchronizing signal generationmeans for generating and providing to said drive means a firstsynchronizing signal for emitting said pulsed laser beam on said signalrecording plane in reproduction, wherein said first synchronizing signalis generated in synchronization with a signal reproduced from saidsignal recording plane by said pulsed laser beam; wherein said pulsedlaser beam is emitted in reproduction by rotating the direction ofpolarization of the laser beam transmitting through said polarizationdirection switching means in synchronization with said firstsynchronizing signal.